A power semiconductor element performs on/off control of a main current by a voltage or current signal applied to a control electrode. An MOSFET (Metal Oxide Semiconductor Field Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor) and the like are power elements of a voltage drive type that perform the on/off control according to the voltage signal, A bipolar transistor and the like are power elements of a current drive type that perform the on/off control according to the current signal.
In many of such power elements, a reverse bias is applied between a control electrode (a gate or base electrode) and a main electrode (a source or emitter electrode) when the element is to be switched from the on state to the off state. In the power element of the voltage drive type, the above bias is applied for ensuring an off state without being influenced by noises. In the element of the current drive type, the above bias is applied for reducing a turn-off time of a main current. The following first and second prior arts are known as the drive circuits for applying the reverse bias.
The first prior art uses two power supplies for forward and reverse biases, respectively (see Japanese Patent Laying-Open No. 07-131971 (Patent Document 1), column [0003] in specification and FIG. 8). When the IGBT is used as the power element, an on-transistor used as a switch element for turn-on, on-resistance, an off-transistor used as a switch element for turn-off, and off-resistance are connected in series. These are connected to a main gate power supply formed of a series connection of a forward bias power supply and a reverse bias power supply, and a connection point between the on-resistance and the off-resistance is connected to a gate of the IGBT so that a gate drive circuit is formed. When the on-transistor is turned on, a forward bias current flows from the forward bias power supply through the on-transistor, on-resistance, gate of the IGBT and emitter of the IGBT to the forward bias power supply, and thereby turns on the IGBT. When the off-transistor is turned on, a reverse bias current flows from the reverse bias power supply through the emitter of the IGBT, gate of the IGBT, off-resistance and off-transistor to the reverse bias power supply, and thereby turns off the IGBT.
In the second prior art, a capacitor for charging is employed together with a single power supply, and is used as a pseudo-power supply (Japanese Patent Laying-Open No. 09-140122 (Patent Document 2)). In this prior art, a diode is connected between base and emitter terminals of a P-type transistor, and a resistance is connected between base and collector terminals thereof. This P-type transistor is connected in parallel between input terminals of a pulse power supply. The emitter terminal of the P-type transistor is connected to a gate terminal of an IGBT via a first resistance, and a collector terminal of the P-type transistor is connected via a parallel circuit formed of a capacitor and a constant-voltage diode to a second resistance connected to the gate terminal of the IGBT. Thereby, a gate drive circuit is formed. When the pulse power supply applies a positive bias to the gate terminal of the IGBT via the diode and the first resistance, the capacitor is likewise charged through the second resistance. When the positive bias is off, the P-type transistor is on so that the charges accumulated in the capacitor are discharged through the P-type transistor and the second resistance, and the reverse bias is applied to the gate terminal of the IGBT.    Patent Document 1: Japanese Patent Laying-Open No. 07-131971    Patent Document 2: Japanese Patent Laying-Open No. 09-140122